Search found 2876 matches

by tomfid
Tue Jan 21, 2020 5:41 am
Forum: Vensim
Topic: demand and supply at price
Replies: 29
Views: 2755

Re: demand and supply at price

If you step through the elements of "priority vector" you will find that the first element has [duration,supplier,ptype] and the next has [duration,supplier,pprofile]. That doesn't work because pprofile is the range. If you replace it with the element ppriority it should work.
by tomfid
Mon Jan 20, 2020 5:37 pm
Forum: Vensim
Topic: Vensim optimization / Powell’s “Hill Climbing” algorithm
Replies: 1
Views: 120

Re: Vensim optimization / Powell’s “Hill Climbing” algorithm

It's fairly close to the Numerical Recipes version: http://www.nrbook.com/a/bookcpdf.php Powell's method (and any other hill climber) will find only a single local optimum, unless you do a grid or random variation of the starting point. Then the probability of discovery of additional optima is propo...
by tomfid
Mon Jan 20, 2020 5:19 pm
Forum: Vensim
Topic: demand and supply at price
Replies: 29
Views: 2755

Re: demand and supply at price

It should work as you've described, if the dimensionality of demand is reduced to 1. The demand priority should be greater than the max supply priority, to ensure that demand is always fulfilled. If you want stepwise allocations, the width parameter needs to be narrow.
by tomfid
Mon Jan 20, 2020 4:25 pm
Forum: Vensim
Topic: Scale of Y-axis in standard Vensim graphs
Replies: 4
Views: 135

Re: Scale of Y-axis in standard Vensim graphs

You can do this temporarily by dragging to zoom in on a regular graph. The setting isn't saved though.
by tomfid
Sat Jan 18, 2020 12:15 am
Forum: Vensim
Topic: Use of the TREND and DELAY functions in Vensim
Replies: 5
Views: 199

Re: Use of the TREND and DELAY functions in Vensim

In that case, you don't want the TREND function at all. You want something like a stock of Workforce, with an inflow of hiring or workforce growth or similar, where the inflow (people/month) equals the stock (people) multiplied by a growth rate (fraction per month, e.g. .005).
by tomfid
Fri Jan 17, 2020 6:04 pm
Forum: Vensim
Topic: Use of the TREND and DELAY functions in Vensim
Replies: 5
Views: 199

Re: Use of the TREND and DELAY functions in Vensim

See also:

Sterman, John D. 1987. Expectation Formation in Behavioral Simulation Models. Behavioral Science, 32:190-211.

Sterman, John D. 2000, Business Dynamics, Boston, MA: McGraw-Hill. Chapter 16.

And:

https://metasd.com/2011/04/delay-sandbox/
by tomfid
Fri Jan 17, 2020 3:26 pm
Forum: Vensim
Topic: Use of the TREND and DELAY functions in Vensim
Replies: 5
Views: 199

Re: Use of the TREND and DELAY functions in Vensim

There's a sample model for each, attached to the function's topic in the Help system.
by tomfid
Wed Jan 15, 2020 3:21 pm
Forum: Vensim
Topic: A question about calculating median
Replies: 6
Views: 250

Re: A question about calculating median

You can report the median with the Stats tool.

If you need it dynamically, you store values in an array and use the VECTOR SORT-type functions to find the midpoint.
by tomfid
Tue Jan 14, 2020 2:29 pm
Forum: Vensim
Topic: qualitative analysis (open end survey questionaire)
Replies: 1
Views: 118

Re: qualitative analysis (open end survey questionaire)

You are correct that Vensim is usually quantitative, but you can use it for pure diagramming tasks. Here's a video tutorial: https://vensim.com/vensim-causal-loop-diagramming/ Another option is to use Kumu.io. I find it slower for making CLDs, but it does have some nice network analysis features. Tu...
by tomfid
Mon Jan 13, 2020 2:49 pm
Forum: Vensim
Topic: User-defined file setup in Sensitivity Simulation - for multiple parameters
Replies: 14
Views: 465

Re: User-defined file setup in Sensitivity Simulation - for multiple parameters

I see. In that case, it does seem sensible to do some kind of combinatorial resampling from the individual columns. Unfortunately, the Vensim file sensitivity isn't set up to do this, but I'll take a look - it might be simple to implement.
by tomfid
Mon Jan 13, 2020 1:56 pm
Forum: Vensim
Topic: Does Vensim embed loop intervals?
Replies: 6
Views: 326

Re: Does Vensim embed loop intervals?

Does it make sense to have a small time step in a long-term capacity model? Or if it does, could you then dispense with the array and observe actuals?
by tomfid
Mon Jan 13, 2020 12:28 am
Forum: Vensim
Topic: User-defined file setup in Sensitivity Simulation - for multiple parameters
Replies: 14
Views: 465

Re: User-defined file setup in Sensitivity Simulation - for multiple parameters

Hi - Good question. You are correct, that there isn't a direct way to resample the columns independently in Vensim. You'd have to use R or Python or Excel or something to do that. However.... I think you don't want to resample in that way. If you do, you are in effect sampling each parameter's margi...
by tomfid
Mon Dec 23, 2019 6:53 pm
Forum: Vensim
Topic: Maximum value
Replies: 26
Views: 1914

Re: Maximum value

When I run this model, T2 starts at 341 and reaches 378 at time 2, and remains there after. T2max is the same, because T2 is always nondecreasing. If I override T2 with a Sin wave of period 8, T2max is still OK - it follows the wave to its maximum and then holds that value. If you mean that T2max at...
by tomfid
Mon Dec 23, 2019 5:45 am
Forum: Vensim
Topic: Maximum value
Replies: 26
Views: 1914

Re: Maximum value

This seems to be working fine. T2 and T2max both reach their max values at time 2. What's the problem?
by tomfid
Fri Dec 20, 2019 4:40 pm
Forum: Vensim
Topic: Changing vdf data sources from Excel using dll
Replies: 4
Views: 254

Re: Changing vdf data sources from Excel using dll

Nope - a vdf output from one model can be read directly as input to another model. No conversion needed - you just need to have matching variable names.
by tomfid
Thu Dec 19, 2019 4:57 pm
Forum: Vensim
Topic: Changing vdf data sources from Excel using dll
Replies: 4
Views: 254

Re: Changing vdf data sources from Excel using dll

I would recommend importing the spreadsheet to a vdf and using ordinary data variables rather than the GET VDF functions. Then you can control the data source via SIMULATE>DATA| and ADDDATA.
by tomfid
Thu Dec 19, 2019 4:56 pm
Forum: Vensim
Topic: Bugs?
Replies: 1
Views: 118

Re: Bugs?

1 & 3 are known and will be fixed in the next release. We'll look into #2.
by tomfid
Tue Dec 17, 2019 5:24 pm
Forum: Vensim
Topic: Misterious Calculation not computed
Replies: 4
Views: 169

Re: Misterious Calculation not computed

A possible test would be to compute SUM(Alfa[Sub!]) in a separate variable.
by tomfid
Fri Dec 13, 2019 3:27 pm
Forum: Vensim
Topic: Getting the final value of a stock to another variable
Replies: 5
Views: 221

Re: Getting the final value of a stock to another variable

More importantly, using the value of a variable at FINAL TIME to set the value of another stock at INITIAL TIME is acausal, i.e. it implies effect preceding cause. You can't do that in Vensim by design, because no one really knows the future.
by tomfid
Thu Dec 12, 2019 2:48 am
Forum: Vensim
Topic: Does Vensim embed loop intervals?
Replies: 6
Views: 326

Re: Does Vensim embed loop intervals?

I think the thing to do is to discretize the LDC into a number of segments, represented by an array dimension. Then you can compute utilization of each plant type in each segment. Then the price is the highest-cost plant with nonzero utilization in the segment. (The allocation functions might make t...
by tomfid
Wed Dec 11, 2019 11:59 pm
Forum: Vensim
Topic: Does Vensim embed loop intervals?
Replies: 6
Views: 326

Re: Does Vensim embed loop intervals?

No problem - we can delete one. Is this the better copy?
by tomfid
Wed Dec 11, 2019 11:18 pm
Forum: Ventity
Topic: Time Series Data Runtime Warnings
Replies: 3
Views: 372

Re: Time Series Data Runtime Warnings

Great! At some point, we'll have data entity singletons, so you don't have to bother with creating a key value for something that has only one instance anyway, as here.
by tomfid
Wed Dec 11, 2019 7:21 pm
Forum: Ventity
Topic: Time Series Data Runtime Warnings
Replies: 3
Views: 372

Re: Time Series Data Runtime Warnings

Hi - You're really close. The data is actually OK. You can verify this in the results: vresults.png The issue is the references in the Growth entitytype. You have 3: Grass6 GrassDataID GrassGrow (These can be found in the References tab of the Growth inspector.) GrassDataID is the reference correspo...
by tomfid
Mon Dec 09, 2019 8:04 pm
Forum: Ventity
Topic: Integration method
Replies: 3
Views: 275

Re: Integration method

OK, thanks for the feedback.

The things we'd probably consider, in descending order of priority, would be:
- RK4auto
- RK2/4 fixed
- An implicit method
An Eigenvector method would be cool, but unfortunately not compatible with action scheduling.
by tomfid
Mon Dec 09, 2019 6:32 pm
Forum: Ventity
Topic: Integration method
Replies: 3
Views: 275

Re: Integration method

At some point, yes, but we haven't had any signals from the user base that it's a priority. What are you looking for?